This invention relates generally to the storage and subsequent recovery of thermal energy and more specifically to the storage of thermal energy produced during a compression mode in a Compressed Air Energy Storage (CAES) system and subsequent recovery of such heat energy in an expansion, or power generation mode.
CAES power plants have become effective contributors to a utility's generation mix as a source of peaking or intermediate energy and spinning reserve. CAES plants store off-peak energy from relatively inexpensive energy sources such as coal and nuclear baseload plants by compressing air into storage devices such as underground caverns or reservoirs. Such underground storage can be developed in hard rock, bedded salt, salt dome or aquifer media.
Following off-peak storage, the air is withdrawn from storage, heated, combined with fuel and expanded through expanders, i.e., turbines, to provide needed peaking/intermediate power. Since inexpensive off-peak energy is used compress the air, the need for premium fuels, such as natural gas and imported oil, is reduced by as much as two thirds compared with conventional gas turbines.
Compressors and turbines in CAES plants are each connected to a generator/motor device through respective clutches, permitting operation either solely of the compressors or solely of the turbines during appropriate selected time periods. During off-peak periods (i.e., nights and weekends), the compressor train is driven through its clutch by the generator/motor. In this scheme, the generator/motor functions as a motor, drawing power from a power grid. The compressed air is then cooled and delivered to underground storage.
During peak or intermediate periods, with the turbine clutch engaged, air is withdrawn from storage and then heated and expanded through a turbine train to provide power by driving the generator/motor. In this scheme, the generator/motor functions as a generator, providing power to a power grid. To improve the CAES heat rate, waste heat from a low pressure turbine exhaust is used to preheat high pressure turbine inlet air in a recuperator.
For a more complete discussion of CAES systems, see Nakhamkin, M. et. al. "Compressed Air Energy Storage: Plant Integration, Turbomachinery Development", ASME International Gas Turbine Symposium and Exhibition, Beijing, Peoples' Republic of China, 1985 and Nakhamkin, M. et. al. "Compressed Air Energy Storage (CAES): Overview, Performance and Cost Data for 25MW to 220MW Plants", Joint Power Generation Conference, Toronto, Canada, 1984, both incorporated herein by reference.
The compression process of the CAES plant is characterized by a much higher overall compression ratio than that for conventional gas turbines. This requires multistage compression with intercoolers in order to improve CAES plant efficiency.
It has been recognized that heat rejected in intercoolers utilized to cool compressed air during the compression process typically represents over ninety percent of the overall energy consumed and conventionally wasted. It is therefore desirable to utilize this conventionally wasted heat in order to improve the overall efficiency of the power plant.
Although devices such as that disclosed in U.S. Pat. No. 4,100,745 which store heat produced by a baseload power plant for subsequent use in a peaking/intermediate power plant are known, it is believed that no adequate system exists as a part of the CAES plant cycle for storing heat produced by a peaking/intermediate plant during its compression mode for subsequent use in the same plant during its expansion mode.
An additional deficiency of known systems which require combustion of fossil fuels is the production of nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2), collectively referred to as NO.sub.x which is a by-product of the combustion of virtually all fossil fuels. In the past, the quantities of such oxides of nitrogen were believed to be insufficient to adversely affect the environment. However, present concern with the efficiency of the combustion process coupled with a realization that such oxides of nitrogen are key constituents in a complex photochemical oxidant reaction with sunlight which produces smog have culminated in extensive regulations by both state and federal authorities.
Present use of NO.sub.x formation inhibitation devices and techniques as well as removal equipment requires the use of specialized dedicated equipment, resulting in increased consumer power costs.